This invention relates to methods of producing densified composite ceramic articles. More particularly, it is concerned with a method of producing dense, polycrystalline silicon nitride articles having thin surface layers of controlled composition.
Densified composite ceramic articles based upon silicon nitride have been the subject of considerable study. Powder mixtures predominantly comprising silicon nitride, when consolidated to densities approaching the theoretical maximum, result in the production of densified ceramic articles having desirable high temperature properties. These articles have refractory character, high temperature creep resistance, thermal shock resistance, and strength which make them ideal for many high temperature applications.
To produce densified composite ceramics from powder mixtures, three methods of consolidation generally have been employed: hot pressing, hot isostatic pressing, and so-called pressureless sintering.
Hot pressing involves the simultaneous application of heat and mechanical pressure to the powder mixture at temperatures high enough to cause sintering. Typically, pressures of several tons per square inch are applied uniaxially to the powder mixture, usually by means of a ram press.
In the hot isostatic pressing method, the powder mixture is placed in a non-permeable deformable container to which heat and pressure are applied at temperatures high enough to cause sintering. In this method, the pressure is applied equally in all directions on the powder mixture, usually by means of a pressurized fluid.
Although sintering of the powder mixture occurs during both of the foregoing processes, the term "sintering" when used in connection with processes for densifying powder mixtures, generally connotes the process of densifying a pre-pressed powder compact without the aid of pressure applied external to the compact which drives the powder particles together.
Sintering is the preferred method of densifying composite ceramic articles because of both the cost and complexity of equipment needed for other methods and because of the difficulty of producing some intricately shaped ceramic articles by diamond grinding following hot pressing or hot isostatic pressing.
It is known in the art to add other materials to silicon nitride to aid in its sintering and to confer valuable high temperature properties upon the finished article. Densification aids which have been used for this purpose include aluminum oxide, magnesium oxide, yttrium oxide, and the oxides of the rare earth elements.
Although pressureless sintering of silicon nitride containing additives is an attractive process for producing densified composite ceramics, the method has some drawbacks. Decomposition or volatilization of silicon nitride or of other components of the initial powder mixture during sintering can often occur at the elevated temperatures required.